Remote Control Transceiver (RMT)

Introduction

The RMT (Remote Control Transceiver) peripheral was designed to act as an infrared transceiver. However, due to the flexibility of its data format, the functionality of RMT can be extended to a versatile and general purpose transceiver. From the perspective of network layering, the RMT hardware contains both physical and data link layer. The physical layer defines the communication media and bit signal representation. The data link layer defines the format of an RMT frame. The minimal data unit in the frame is called RMT symbol, which is represented by rmt_symbol_word_t in the driver.

ESP32-S2 contains multiple channels in the RMT peripheral. 1 Each channel can be configured as either transmitter or receiver, independently.

Typically, the RMT peripheral can be used in the following scenarios:

  • Transmit or receive infrared signals, with any IR protocols, e.g. NEC

  • General purpose sequence generator

  • Transmit signals in a hardware controlled loop, with finite or infinite number of times

  • Multi-channel simultaneous transmission

  • Modulate the carrier to the output signal or demodulate the carrier from the input signal

Layout of RMT Symbols

The RMT hardware defines data in its own pattern – the RMT symbol. Each symbol consists of two pairs of two values. The first value in a pair describes the signal duration in RMT ticks and is 15 bits long. The second provides the signal level (high or low) and is contained in a single bit, as shown below:

Structure of RMT symbols (L - signal level)

RMT Transmitter Overview

The data path and control path of an RMT TX channel is illustrated in the figure below:

RMT Transmitter Overview

The driver will encode user’s data into RMT data format, then the RMT transmitter can generate the waveforms according to the encoding artifacts. It is also possible to modulate a high frequency carrier signal before being routed to a GPIO pad.

RMT Receiver Overview

The data path and control path of an RMT RX channel is illustrated in the figure below:

RMT Receiver Overview

The RMT receiver can sample incoming signals into RMT data format, and store the data in memory. It’s feasible to tell the receiver the basic characteristics of the incoming signal, so that the signal’s stop condition can be recognized, and signal glitches and noise can be filtered out. The RMT peripheral also supports demodulating the high frequency carrier from the base signal.

Functional Overview

Description of the RMT functionality is divided into the following sections:

  • Resource Allocation - covers how to allocate RMT channels with properly set of configurations. It also covers how to recycle the resources when they finished working.

  • Carrier Modulation and Demodulation - describes how to modulate carrier for TX channel and demodulate carrier for RX channel.

  • Register Event Callbacks - covers how to hook user specific code to RMT channel specific events.

  • Enable and Disable channel - shows how to enable and disable the RMT channel.

  • Initiate TX Transaction - describes the steps to initiate a transaction for TX channel.

  • Initiate RX Transaction - describes the steps to initiate a transaction for RX channel.

  • Multiple Channels Simultaneous Transmission - describes how to collect multiple channels into a sync group and start transaction at the same time.

  • RMT Encoder - focuses on how to write a customized encoder in a combination way, with the help of the primitive encoders provided by the driver.

  • Power Management - describes how different source clock will affect power consumption.

  • IRAM Safe - describes tips on how to make the RMT interrupt work better along with a disabled cache.

  • Thread Safety - lists which APIs are guaranteed to be thread safe by the driver.

  • Kconfig Options - lists the supported Kconfig options that can bring different effects to the driver.

Resource Allocation

Both RMT TX and RX channels are represented by rmt_channel_handle_t in the driver. The available channels are managed in a resource pool, which will hand out a free channel on request.

Install RMT TX Channel

To install an RMT TX channel, there’s a configuration structure that needs to be given in advance: rmt_tx_channel_config_t:

Once the rmt_tx_channel_config_t structure is populated with mandatory parameters, users can call rmt_new_tx_channel() to allocate and initialize a TX channel. This function will return an RMT channel handle if it runs correctly. Specifically, when there are no more free channels in the RMT resource pool, this function will return ESP_ERR_NOT_FOUND error. If some feature (e.g. DMA backend) is not supported by hardware, it will return ESP_ERR_NOT_SUPPORTED error.

rmt_channel_handle_t tx_chan = NULL;
rmt_tx_channel_config_t tx_chan_config = {
    .clk_src = RMT_CLK_SRC_DEFAULT,       // select source clock
    .gpio_num = 0,                    // GPIO number
    .mem_block_symbols = 64,          // memory block size, 64 * 4 = 256Bytes
    .resolution_hz = 1 * 1000 * 1000, // 1MHz tick resolution, i.e. 1 tick = 1us
    .trans_queue_depth = 4,           // set the number of transactions that can pend in the background
    .flags.invert_out = false,        // don't invert output signal
    .flags.with_dma = false,          // don't need DMA backend
};
ESP_ERROR_CHECK(rmt_new_tx_channel(&tx_chan_config, &tx_chan));

Install RMT RX Channel

To install an RMT RX channel, there’s a configuration structure that needs to be given in advance: rmt_rx_channel_config_t:

  • rmt_rx_channel_config_t::gpio_num sets the GPIO number used by the receiver.

  • rmt_rx_channel_config_t::clk_src selects the source clock for the RMT channel. The available clocks are listed in rmt_clock_source_t. Note that, the selected clock will also be used by other channels, which means user should ensure this configuration is same when allocating other channels, regardless of TX or RX. For the effect on power consumption of different clock source, please refer to Power Management section.

  • rmt_rx_channel_config_t::resolution_hz sets the resolution of the internal tick counter. The timing parameter of RMT signal is calculated based on this tick.

  • rmt_rx_channel_config_t::mem_block_symbols sets the size of the dedicated memory block or DMA buffer that used to store RMT encoding artifacts.

  • rmt_rx_channel_config_t::invert_in is used to decide whether to invert the input signals before they going into RMT receiver. The inversion is done by GPIO matrix instead of by the RMT peripheral.

  • rmt_rx_channel_config_t::with_dma is used to indicate if the channel needs a DMA backend. A channel with DMA attached can offload the CPU by a lot. However, DMA backend is not available on all ESP chips, please refer to [TRM] before you enable this option. Or you might encounter ESP_ERR_NOT_SUPPORTED error.

  • rmt_rx_channel_config_t::io_loop_back is for debugging purposes only. It enables both the GPIO’s input and output ability through the GPIO matrix peripheral. Meanwhile, if both TX and RX channels are bound to the same GPIO, then monitoring of the data transmission line can be realized.

Once the rmt_rx_channel_config_t structure is populated with mandatory parameters, users can call rmt_new_rx_channel() to allocate and initialize a RX channel. This function will return an RMT channel handle if it runs correctly. Specifically, when there are no more free channels in the RMT resource pool, this function will return ESP_ERR_NOT_FOUND error. If some feature (e.g. DMA backend) is not supported by hardware, it will return ESP_ERR_NOT_SUPPORTED error.

rmt_channel_handle_t rx_chan = NULL;
rmt_rx_channel_config_t rx_chan_config = {
    .clk_src = RMT_CLK_SRC_DEFAULT,       // select source clock
    .resolution_hz = 1 * 1000 * 1000, // 1MHz tick resolution, i.e. 1 tick = 1us
    .mem_block_symbols = 64,          // memory block size, 64 * 4 = 256Bytes
    .gpio_num = 2,                    // GPIO number
    .flags.invert_in = false,         // don't invert input signal
    .flags.with_dma = false,          // don't need DMA backend
};
ESP_ERROR_CHECK(rmt_new_rx_channel(&rx_chan_config, &rx_chan));

Uninstall RMT Channel

If a previously installed RMT channel is no longer needed, it’s recommended to recycle the resources by calling rmt_del_channel(), which in return allows the underlying hardware to be usable for other purposes.

Carrier Modulation and Demodulation

The RMT transmitter can generate a carrier wave and modulate it onto the base signal. Compared to the base signal, the carrier frequency is usually high. In addition, user can only set the frequency and duty cycle for the carrier. The RMT receiver can demodulate the carrier from the incoming signal. Note that, carrier modulation and demodulation is not supported on all ESP chips, please refer to [TRM] before configuring the carrier, or you might encounter a ESP_ERR_NOT_SUPPORTED error.

Carrier related configurations lie in rmt_carrier_config_t:

备注

For RX channel, we shouldn’t set the carrier frequency exactly to the theoretical value. It’s recommended to leave a tolerance for the carrier frequency. For example, in the snippet below, we set the frequency to 25KHz, instead of the 38KHz that configured on the TX side. The reason is that reflection and refraction will occur when a signal travels through the air, leading to the a distortion on the receiver side.

rmt_carrier_config_t tx_carrier_cfg = {
    .duty_cycle = 0.33,                 // duty cycle 33%
    .frequency_hz = 38000,              // 38KHz
    .flags.polarity_active_low = false, // carrier should modulated to high level
};
// modulate carrier to TX channel
ESP_ERROR_CHECK(rmt_apply_carrier(tx_chan, &tx_carrier_cfg));

rmt_carrier_config_t rx_carrier_cfg = {
    .duty_cycle = 0.33,                 // duty cycle 33%
    .frequency_hz = 25000,              // 25KHz carrier, should be smaller than transmitter's carrier frequency
    .flags.polarity_active_low = false, // the carrier is modulated to high level
};
// demodulate carrier from RX channel
ESP_ERROR_CHECK(rmt_apply_carrier(rx_chan, &rx_carrier_cfg));

Register Event Callbacks

When an RMT channel finishes transmitting or receiving, a specific event will be generated and notify the CPU by interrupt. If you have some function that needs to be called when those events occurred, you can hook your function to the ISR (Interrupt Service Routine) by calling rmt_tx_register_event_callbacks() and rmt_rx_register_event_callbacks() for TX and RX channel respectively. Since the registered callback functions are called in the interrupt context, user should ensure the callback function doesn’t attempt to block (e.g. by making sure that only FreeRTOS APIs with ISR suffix are called from within the function). The callback function has a boolean return value, to tell the caller whether a high priority task is woke up by it.

TX channel supported event callbacks are listed in the rmt_tx_event_callbacks_t:

RX channel supported event callbacks are listed in the rmt_rx_event_callbacks_t:

User can save own context in rmt_tx_register_event_callbacks() and rmt_rx_register_event_callbacks() as well, via the parameter user_data. The user data will be directly passed to each callback function.

In the callback function, users can fetch the event specific data that is filled by the driver in the edata. Note that the edata pointer is only valid for the duration of the callback.

The TX done event data is defined in rmt_tx_done_event_data_t:

The RX complete event data is defined in rmt_rx_done_event_data_t:

Enable and Disable channel

rmt_enable() must be called in advanced before transmitting or receiving RMT symbols. For transmitters, enabling a channel will enable a specific interrupt and prepare the hardware to dispatch transactions. For RX channels, enabling a channel will enable an interrupt, but the receiver is not started during this time, as it has no idea about the characteristics of the incoming signals. The receiver will be started in rmt_receive().

rmt_disable() does the opposite work by disabling the interrupt and clearing pending status. The transmitter and receiver will be disabled as well.

ESP_ERROR_CHECK(rmt_enable(tx_chan));
ESP_ERROR_CHECK(rmt_enable(rx_chan));

Initiate TX Transaction

RMT is a special communication peripheral as it’s unable to transmit raw byte streams like SPI and I2C. RMT can only send data in its own format rmt_symbol_word_t. However, the hardware doesn’t help to convert the user data into RMT symbols, this can only be done in software — by the so-called RMT Encoder. The encoder is responsible for encoding user data into RMT symbols and then write to RMT memory block or DMA buffer. For how to create an RMT encoder, please refer to RMT Encoder.

Once we got an encoder, we can initiate a TX transaction by calling rmt_transmit(). This function takes several positional parameters like channel handle, encoder handle, payload buffer. Besides that, we also need to provide a transmission specific configuration in rmt_transmit_config_t:

  • rmt_transmit_config_t::loop_count sets the number of transmission loop. After the transmitter finished one round of transmission, it can restart the same transmission again if this value is not set to zero. As the loop is controlled by hardware, the RMT channel can be used to generate many periodic sequences at the cost of a very little CPU intervention. Specially, setting rmt_transmit_config_t::loop_count to -1 means an infinite loop transmission. In this situation, the channel won’t stop until manually call of rmt_disable(). And the trans done event won’t be generated as well. If rmt_transmit_config_t::loop_count is set to a positive number, the trans done event won’t be generated until target number of loop transmission have finished. Note that, the loop transmit feature is not supported on all ESP chips, please refer to [TRM] before you configure this option. Or you might encounter ESP_ERR_NOT_SUPPORTED error.

  • rmt_transmit_config_t::eot_level sets the output level when the transmitter finishes working or stops working by calling rmt_disable().

备注

There’s a limitation in the transmission size if the rmt_transmit_config_t::loop_count is set to non-zero (i.e. to enable the loop feature). The encoded RMT symbols should not exceed the capacity of RMT hardware memory block size. Or you might see error message like encoding artifacts can't exceed hw memory block for loop transmission. If you have to start a large transaction by loop, you can try either:

Internally, rmt_transmit() will construct a transaction descriptor and send to a job queue, which will be dispatched in the ISR. So it is possible that the transaction is not started yet when rmt_transmit() returns. To ensure all pending transaction to complete, user can use rmt_tx_wait_all_done().

Multiple Channels Simultaneous Transmission

In some real-time control applications, we don’t want any time drift in between when startup multiple TX channels. For example, to make two robotic arms move simultaneously. The RMT driver can help to manage this by creating a so-called Sync Manager. The sync manager is represented by rmt_sync_manager_handle_t in the driver. The procedure of RMT sync transmission is shown as follows:

RMT TX Sync

RMT TX Sync

Install RMT Sync Manager

To create a sync manager, user needs to tell which channels are going to be managed in the rmt_sync_manager_config_t:

rmt_new_sync_manager() can return a manager handle on success. This function could also fail due to various errors such as invalid arguments, etc. Specially, when the sync manager has been installed before, and there’re no hardware resources to create another manager, this function will report ESP_ERR_NOT_FOUND error. In addition, if the sync manager is not supported by the hardware, it will report ESP_ERR_NOT_SUPPORTED error. Please refer to [TRM] before using the sync manager feature.

Start Transmission Simultaneously

For any managed TX channel, it won’t start the machine until all the channels in the rmt_sync_manager_config_t::tx_channel_array are called with rmt_transmit(). Before that, the channel is just put in a waiting state. Different channel usually take different time to finish the job if the transaction is different, which results in a loss of sync. So user needs to call rmt_sync_reset() to pull the channels back to the starting line again before restarting a simultaneous transmission.

Calling rmt_del_sync_manager() can recycle the sync manager and enable the channels to initiate transactions independently afterwards.

rmt_channel_handle_t tx_channels[2] = {NULL}; // declare two channels
int tx_gpio_number[2] = {0, 2};
// install channels one by one
for (int i = 0; i < 2; i++) {
    rmt_tx_channel_config_t tx_chan_config = {
        .clk_src = RMT_CLK_SRC_DEFAULT,       // select source clock
        .gpio_num = tx_gpio_number[i],    // GPIO number
        .mem_block_symbols = 64,          // memory block size, 64 * 4 = 256Bytes
        .resolution_hz = 1 * 1000 * 1000, // 1MHz resolution
        .trans_queue_depth = 1,           // set the number of transactions that can pend in the background
    };
    ESP_ERROR_CHECK(rmt_new_tx_channel(&tx_chan_config, &tx_channels[i]));
}
// install sync manager
rmt_sync_manager_handle_t synchro = NULL;
rmt_sync_manager_config_t synchro_config = {
    .tx_channel_array = tx_channels,
    .array_size = sizeof(tx_channels) / sizeof(tx_channels[0]),
};
ESP_ERROR_CHECK(rmt_new_sync_manager(&synchro_config, &synchro));

ESP_ERROR_CHECK(rmt_transmit(tx_channels[0], led_strip_encoders[0], led_data, led_num * 3, &transmit_config));
// tx_channels[0] won't start transmission until call of `rmt_transmit()` for tx_channels[1] returns
ESP_ERROR_CHECK(rmt_transmit(tx_channels[1], led_strip_encoders[1], led_data, led_num * 3, &transmit_config));

Initiate RX Transaction

As also discussed in the Enable and Disable channel, the RX channel still doesn’t get ready to receive RMT symbols even user calls rmt_enable(). User needs to specify the basic characteristics of the incoming signals in rmt_receive_config_t:

  • rmt_receive_config_t::signal_range_min_ns specifies the minimal valid pulse duration (either high or low level). A pulse whose width is smaller than this value will be treated as glitch and ignored by the hardware.

  • rmt_receive_config_t::signal_range_max_ns specifies the maximum valid pulse duration (either high or low level). A pulse whose width is bigger than this value will be treated as Stop Signal, and the receiver will generate receive complete event immediately.

The RMT receiver will start the RX machine after user calls rmt_receive() with the provided configuration above. Note that, this configuration is transaction specific, which means, to start a new round of reception, user needs to sets the rmt_receive_config_t again. The receiver saves the incoming signals into its internal memory block or DMA buffer, in the format of rmt_symbol_word_t.

Due to the limited size of memory block, the RMT receiver can only save short frames whose length is not longer than the memory block capacity. Long frames will be truncated by the hardware, and the driver will report an error message: hw buffer too small, received symbols truncated.

The copy destination should be provided in the buffer parameter of rmt_receive() function. If this buffer size is not sufficient, the receiver can continue to work but later incoming symbols will be dropped and report an error message: user buffer too small, received symbols truncated. Please take care of the lifecycle of the buffer parameter, user shouldn’t recycle the buffer before the receiver finished or stopped working.

The receiver will be stopped by the driver when it finishes working (i.e. received a signal whose duration is bigger than rmt_receive_config_t::signal_range_max_ns). User needs to call rmt_receive() again to restart the receiver, is necessary. User can get the received data in the rmt_rx_event_callbacks_t::on_recv_done callback. See also Register Event Callbacks for more information.

static bool example_rmt_rx_done_callback(rmt_channel_handle_t channel, rmt_rx_done_event_data_t *edata, void *user_data)
{
    BaseType_t high_task_wakeup = pdFALSE;
    TaskHandle_t task_to_notify = (TaskHandle_t)user_data;
    // send the received RMT symbols to the parser task
    xTaskNotifyFromISR(task_to_notify, (uint32_t)edata, eSetValueWithOverwrite, &high_task_wakeup);
    // return whether any task is woken up
    return high_task_wakeup == pdTRUE;
}

TaskHandle_t cur_task = xTaskGetCurrentTaskHandle();
rmt_rx_event_callbacks_t cbs = {
    .on_recv_done = example_rmt_rx_done_callback,
};
ESP_ERROR_CHECK(rmt_rx_register_event_callbacks(rx_channel, &cbs, cur_task));

// the following timing requirement is based on NEC protocol
rmt_receive_config_t receive_config = {
    .signal_range_min_ns = 1250,     // the shortest duration for NEC signal is 560us, 1250ns < 560us, valid signal won't be treated as noise
    .signal_range_max_ns = 12000000, // the longest duration for NEC signal is 9000us, 12000000ns > 9000us, the receive won't stop early
};

rmt_symbol_word_t raw_symbols[64]; // 64 symbols should be sufficient for a standard NEC frame
// ready to receive
ESP_ERROR_CHECK(rmt_receive(rx_channel, raw_symbols, sizeof(raw_symbols), &receive_config));
// wait for RX done signal
rmt_rx_done_event_data_t *rx_data = NULL;
xTaskNotifyWait(0x00, ULONG_MAX, (uint32_t *)&rx_data, portMAX_DELAY);
// parse the receive symbols
example_parse_nec_frame(rx_data->received_symbols, rx_data->num_symbols);

RMT Encoder

An RMT encoder is part of the RMT TX transaction, whose responsibility is to generate and write the correct RMT symbols into hardware memory (or DMA buffer) at specific time. There’re some special restrictions for an encoding function:

  • An encoding function might be called for several times within a single transaction. This is because the target RMT memory block can’t accommodate all the artifacts at once. We have to use the memory in a ping-pong way, thus the encoding session is divided into multiple parts. This requires the encoder to be stateful.

  • The encoding function is running in the ISR context. To speed up the encoding session, it’s high recommend to put the encoding function into IRAM. This can also avoid the cache miss during encoding.

To help get started with RMT driver faster, some commonly used encoders are provided out-of-the box. They can either work alone or chained together into a new encoder. See also Composite Pattern for the principle behind. The driver has defined the encoder interface in rmt_encoder_t, it contains the following functions:

  • rmt_encoder_t::encode is the fundamental function of an encoder. This is where the encoding session happens. Please note, the rmt_encoder_t::encode function might be called for multiple times within a single transaction. The encode function should return the state of current encoding session. The supported states are listed in the rmt_encode_state_t. If the result contains RMT_ENCODING_COMPLETE, it means the current encoder has finished work. If the result contains RMT_ENCODING_MEM_FULL, we need to yield from current session, as there’s no space to save more encoding artifacts.

  • rmt_encoder_t::reset should reset the encoder state back to initial. The RMT encoder is stateful, if RMT transmitter stopped manually without its corresponding encoder being reset, then the following encoding session can be wrong. This function is also called implicitly in rmt_disable().

  • rmt_encoder_t::del function should free the resources allocated by the encoder.

Copy Encoder

A copy encoder is created by calling rmt_new_copy_encoder(). Copy encoder’s main functionality is to copy the RMT symbols from user space into the driver layer. It’s usually used to encode const data (i.e. data won’t change at runtime after initialization), for example, the leading code in the IR protocol.

A configuration structure rmt_copy_encoder_config_t should be provided in advance before calling rmt_new_copy_encoder(). Currently, this configuration is reserved for future expansion.

Bytes Encoder

A bytes encoder is created by calling rmt_new_bytes_encoder(). Bytes encoder’s main functionality is to convert the user space byte stream into RMT symbols dynamically. It’s usually used to encode dynamic data, for example, the address and command fields in the IR protocol.

A configuration structure rmt_bytes_encoder_config_t should be provided in advance before calling rmt_new_bytes_encoder():

Besides the primitive encoders provided by the driver, user can implement his own encoder by chaining the existing encoders together. A common encoder chain is shown as follows:

RMT Encoder Chain

Customize RMT Encoder for NEC Protocol

In this section, we will demonstrate on how to write an NEC encoder. The NEC IR protocol uses pulse distance encoding of the message bits. Each pulse burst is 562.5µs in length, logical bits are transmitted as follows. It is worth mentioning, the bytes of data bits are sent least significant bit first.

  • Logical 0: a 562.5µs pulse burst followed by a 562.5µs space, with a total transmit time of 1.125ms

  • Logical 1: a 562.5µs pulse burst followed by a 1.6875ms space, with a total transmit time of 2.25ms

When a key is pressed on the remote controller, the message transmitted consists of the following, in order:

IR NEC Frame

IR NEC Frame

  • 9ms leading pulse burst (also called the “AGC pulse”)

  • 4.5ms space

  • 8-bit address for the receiving device

  • 8-bit logical inverse of the address

  • 8-bit command

  • 8-bit logical inverse of the command

  • a final 562.5µs pulse burst to signify the end of message transmission

Then we can construct the NEC rmt_encoder_t::encode function in the same order, for example:

// IR NEC scan code representation
typedef struct {
    uint16_t address;
    uint16_t command;
} ir_nec_scan_code_t;

// construct a encoder by combining primitive encoders
typedef struct {
    rmt_encoder_t base;           // the base "class", declares the standard encoder interface
    rmt_encoder_t *copy_encoder;  // use the copy_encoder to encode the leading and ending pulse
    rmt_encoder_t *bytes_encoder; // use the bytes_encoder to encode the address and command data
    rmt_symbol_word_t nec_leading_symbol; // NEC leading code with RMT representation
    rmt_symbol_word_t nec_ending_symbol;  // NEC ending code with RMT representation
    int state; // record the current encoding state (i.e. we're in which encoding phase)
} rmt_ir_nec_encoder_t;

static size_t rmt_encode_ir_nec(rmt_encoder_t *encoder, rmt_channel_handle_t channel, const void *primary_data, size_t data_size, rmt_encode_state_t *ret_state)
{
    rmt_ir_nec_encoder_t *nec_encoder = __containerof(encoder, rmt_ir_nec_encoder_t, base);
    rmt_encode_state_t session_state = 0;
    rmt_encode_state_t state = 0;
    size_t encoded_symbols = 0;
    ir_nec_scan_code_t *scan_code = (ir_nec_scan_code_t *)primary_data;
    rmt_encoder_handle_t copy_encoder = nec_encoder->copy_encoder;
    rmt_encoder_handle_t bytes_encoder = nec_encoder->bytes_encoder;
    switch (nec_encoder->state) {
    case 0: // send leading code
        encoded_symbols += copy_encoder->encode(copy_encoder, channel, &nec_encoder->nec_leading_symbol,
                                                sizeof(rmt_symbol_word_t), &session_state);
        if (session_state & RMT_ENCODING_COMPLETE) {
            nec_encoder->state = 1; // we can only switch to next state when current encoder finished
        }
        if (session_state & RMT_ENCODING_MEM_FULL) {
            state |= RMT_ENCODING_MEM_FULL;
            goto out; // yield if there's no free space to put other encoding artifacts
        }
    // fall-through
    case 1: // send address
        encoded_symbols += bytes_encoder->encode(bytes_encoder, channel, &scan_code->address, sizeof(uint16_t), &session_state);
        if (session_state & RMT_ENCODING_COMPLETE) {
            nec_encoder->state = 2; // we can only switch to next state when current encoder finished
        }
        if (session_state & RMT_ENCODING_MEM_FULL) {
            state |= RMT_ENCODING_MEM_FULL;
            goto out; // yield if there's no free space to put other encoding artifacts
        }
    // fall-through
    case 2: // send command
        encoded_symbols += bytes_encoder->encode(bytes_encoder, channel, &scan_code->command, sizeof(uint16_t), &session_state);
        if (session_state & RMT_ENCODING_COMPLETE) {
            nec_encoder->state = 3; // we can only switch to next state when current encoder finished
        }
        if (session_state & RMT_ENCODING_MEM_FULL) {
            state |= RMT_ENCODING_MEM_FULL;
            goto out; // yield if there's no free space to put other encoding artifacts
        }
    // fall-through
    case 3: // send ending code
        encoded_symbols += copy_encoder->encode(copy_encoder, channel, &nec_encoder->nec_ending_symbol,
                                                sizeof(rmt_symbol_word_t), &session_state);
        if (session_state & RMT_ENCODING_COMPLETE) {
            nec_encoder->state = 0; // back to the initial encoding session
            state |= RMT_ENCODING_COMPLETE; // telling the caller the NEC encoding has finished
        }
        if (session_state & RMT_ENCODING_MEM_FULL) {
            state |= RMT_ENCODING_MEM_FULL;
            goto out; // yield if there's no free space to put other encoding artifacts
        }
    }
out:
    *ret_state = state;
    return encoded_symbols;
}

A full sample code can be found in peripherals/rmt/ir_nec_transceiver. In the above snippet, we use a switch-case plus several goto statements to implement a state machine . With this pattern, user can construct a lot more complex IR protocols.

Power Management

When power management is enabled (i.e. CONFIG_PM_ENABLE is on), the system will adjust the APB frequency before going into light sleep, thus potentially changing the resolution of RMT internal counter.

However, the driver can prevent the system from changing APB frequency by acquiring a power management lock of type ESP_PM_APB_FREQ_MAX. Whenever user creates an RMT channel that has selected RMT_CLK_SRC_APB as the clock source, the driver will guarantee that the power management lock is acquired after the channel enabled by rmt_enable(). Likewise, the driver releases the lock after rmt_disable() is called for the same channel. This also reveals that the rmt_enable() and rmt_disable() should appear in pairs.

If the channel clock source is selected to others like RMT_CLK_SRC_XTAL, then the driver won’t install power management lock for it, which is more suitable for a low power application as long as the source clock can still provide sufficient resolution.

IRAM Safe

By default, the RMT interrupt will be deferred when the Cache is disabled for reasons like writing/erasing the main Flash. Thus the transaction done interrupt will not get executed in time, which is not expected in a real-time application. What’s worse, when the RMT transaction relies on ping-pong interrupt to successively encode or copy RMT symbols, such delayed response can lead to an unpredictable result.

There’s a Kconfig option CONFIG_RMT_ISR_IRAM_SAFE that will:

  1. Enable the interrupt being serviced even when cache is disabled

  2. Place all functions that used by the ISR into IRAM 2

  3. Place driver object into DRAM (in case it’s mapped to PSRAM by accident)

This Kconfig option will allow the interrupt to run while the cache is disabled but will come at the cost of increased IRAM consumption.

Thread Safety

The factory function rmt_new_tx_channel(), rmt_new_rx_channel() and rmt_new_sync_manager() are guaranteed to be thread safe by the driver, which means, user can call them from different RTOS tasks without protection by extra locks. Other functions that take the rmt_channel_handle_t and rmt_sync_manager_handle_t as the first positional parameter, are not thread safe. which means the user should avoid calling them from multiple tasks.

Kconfig Options

Application Examples

API Reference

Functions

esp_err_t rmt_new_tx_channel(const rmt_tx_channel_config_t *config, rmt_channel_handle_t *ret_chan)

Create a RMT TX channel.

参数
  • config[in] TX channel configurations

  • ret_chan[out] Returned generic RMT channel handle

返回

  • ESP_OK: Create RMT TX channel successfully

  • ESP_ERR_INVALID_ARG: Create RMT TX channel failed because of invalid argument

  • ESP_ERR_NO_MEM: Create RMT TX channel failed because out of memory

  • ESP_ERR_NOT_FOUND: Create RMT TX channel failed because all RMT channels are used up and no more free one

  • ESP_ERR_NOT_SUPPORTED: Create RMT TX channel failed because some feature is not supported by hardware, e.g. DMA feature is not supported by hardware

  • ESP_FAIL: Create RMT TX channel failed because of other error

esp_err_t rmt_transmit(rmt_channel_handle_t tx_channel, rmt_encoder_handle_t encoder, const void *payload, size_t payload_bytes, const rmt_transmit_config_t *config)

Transmit data by RMT TX channel.

备注

This function will construct a transaction descriptor and push to a queue. The transaction will not start immediately until it’s dispatched in the ISR. If there’re too many transactions pending in the queue, this function will block until the queue has free space.

备注

The data to be transmitted will be encoded into RMT symbols by the specific encoder.

参数
  • tx_channel[in] RMT TX channel that created by rmt_new_tx_channel()

  • encoder[in] RMT encoder that created by various factory APIs like rmt_new_bytes_encoder()

  • payload[in] The raw data to be encoded into RMT symbols

  • payload_bytes[in] Size of the payload in bytes

  • config[in] Transmission specific configuration

返回

  • ESP_OK: Transmit data successfully

  • ESP_ERR_INVALID_ARG: Transmit data failed because of invalid argument

  • ESP_ERR_INVALID_STATE: Transmit data failed because channel is not enabled

  • ESP_ERR_NOT_SUPPORTED: Transmit data failed because some feature is not supported by hardware, e.g. unsupported loop count

  • ESP_FAIL: Transmit data failed because of other error

esp_err_t rmt_tx_wait_all_done(rmt_channel_handle_t tx_channel, int timeout_ms)

Wait for all pending TX transactions done.

备注

This function will block forever if the pending transaction can’t be finished within a limited time (e.g. an infinite loop transaction). See also rmt_disable() for how to terminate a working channel.

参数
  • tx_channel[in] RMT TX channel that created by rmt_new_tx_channel()

  • timeout_ms[in] Wait timeout, in ms. Specially, -1 means to wait forever.

返回

  • ESP_OK: Flush transactions successfully

  • ESP_ERR_INVALID_ARG: Flush transactions failed because of invalid argument

  • ESP_ERR_TIMEOUT: Flush transactions failed because of timeout

  • ESP_FAIL: Flush transactions failed because of other error

esp_err_t rmt_tx_register_event_callbacks(rmt_channel_handle_t tx_channel, const rmt_tx_event_callbacks_t *cbs, void *user_data)

Set event callbacks for RMT TX channel.

备注

User can deregister a previously registered callback by calling this function and setting the callback member in the cbs structure to NULL.

备注

When CONFIG_RMT_ISR_IRAM_SAFE is enabled, the callback itself and functions called by it should be placed in IRAM. The variables used in the function should be in the SRAM as well. The user_data should also reside in SRAM.

参数
  • tx_channel[in] RMT generic channel that created by rmt_new_tx_channel()

  • cbs[in] Group of callback functions

  • user_data[in] User data, which will be passed to callback functions directly

返回

  • ESP_OK: Set event callbacks successfully

  • ESP_ERR_INVALID_ARG: Set event callbacks failed because of invalid argument

  • ESP_FAIL: Set event callbacks failed because of other error

esp_err_t rmt_new_sync_manager(const rmt_sync_manager_config_t *config, rmt_sync_manager_handle_t *ret_synchro)

Create a synchronization manager for multiple TX channels, so that the managed channel can start transmitting at the same time.

备注

All the channels to be managed should be enabled by rmt_enable() before put them into sync manager.

参数
  • config[in] Synchronization manager configuration

  • ret_synchro[out] Returned synchronization manager handle

返回

  • ESP_OK: Create sync manager successfully

  • ESP_ERR_INVALID_ARG: Create sync manager failed because of invalid argument

  • ESP_ERR_NOT_SUPPORTED: Create sync manager failed because it is not supported by hardware

  • ESP_ERR_INVALID_STATE: Create sync manager failed because not all channels are enabled

  • ESP_ERR_NO_MEM: Create sync manager failed because out of memory

  • ESP_ERR_NOT_FOUND: Create sync manager failed because all sync controllers are used up and no more free one

  • ESP_FAIL: Create sync manager failed because of other error

esp_err_t rmt_del_sync_manager(rmt_sync_manager_handle_t synchro)

Delete synchronization manager.

参数

synchro[in] Synchronization manager handle returned from rmt_new_sync_manager()

返回

  • ESP_OK: Delete the synchronization manager successfully

  • ESP_ERR_INVALID_ARG: Delete the synchronization manager failed because of invalid argument

  • ESP_FAIL: Delete the synchronization manager failed because of other error

esp_err_t rmt_sync_reset(rmt_sync_manager_handle_t synchro)

Reset synchronization manager.

参数

synchro[in] Synchronization manager handle returned from rmt_new_sync_manager()

返回

  • ESP_OK: Reset the synchronization manager successfully

  • ESP_ERR_INVALID_ARG: Reset the synchronization manager failed because of invalid argument

  • ESP_FAIL: Reset the synchronization manager failed because of other error

Structures

struct rmt_tx_event_callbacks_t

Group of RMT TX callbacks.

备注

The callbacks are all running under ISR environment

备注

When CONFIG_RMT_ISR_IRAM_SAFE is enabled, the callback itself and functions called by it should be placed in IRAM. The variables used in the function should be in the SRAM as well.

Public Members

rmt_tx_done_callback_t on_trans_done

Event callback, invoked when transmission is finished

struct rmt_tx_channel_config_t

RMT TX channel specific configuration.

Public Members

int gpio_num

GPIO number used by RMT TX channel. Set to -1 if unused

rmt_clock_source_t clk_src

Clock source of RMT TX channel, channels in the same group must use the same clock source

uint32_t resolution_hz

Channel clock resolution, in Hz

size_t mem_block_symbols

Size of memory block, in number of rmt_symbol_word_t, must be an even

size_t trans_queue_depth

Depth of internal transfer queue, increase this value can support more transfers pending in the background

uint32_t invert_out

Whether to invert the RMT channel signal before output to GPIO pad

uint32_t with_dma

If set, the driver will allocate an RMT channel with DMA capability

uint32_t io_loop_back

For debug/test, the signal output from the GPIO will be fed to the input path as well

struct rmt_tx_channel_config_t::[anonymous] flags

TX channel config flags

struct rmt_transmit_config_t

RMT transmit specific configuration.

Public Members

int loop_count

Specify the times of transmission in a loop, -1 means transmitting in an infinite loop

uint32_t eot_level

Set the output level for the “End Of Transmission”

struct rmt_transmit_config_t::[anonymous] flags

Transmit config flags

struct rmt_sync_manager_config_t

Synchronous manager configuration.

Public Members

const rmt_channel_handle_t *tx_channel_array

Array of TX channels that are about to be managed by a synchronous controller

size_t array_size

Size of the tx_channel_array

Functions

esp_err_t rmt_new_rx_channel(const rmt_rx_channel_config_t *config, rmt_channel_handle_t *ret_chan)

Create a RMT RX channel.

参数
  • config[in] RX channel configurations

  • ret_chan[out] Returned generic RMT channel handle

返回

  • ESP_OK: Create RMT RX channel successfully

  • ESP_ERR_INVALID_ARG: Create RMT RX channel failed because of invalid argument

  • ESP_ERR_NO_MEM: Create RMT RX channel failed because out of memory

  • ESP_ERR_NOT_FOUND: Create RMT RX channel failed because all RMT channels are used up and no more free one

  • ESP_ERR_NOT_SUPPORTED: Create RMT RX channel failed because some feature is not supported by hardware, e.g. DMA feature is not supported by hardware

  • ESP_FAIL: Create RMT RX channel failed because of other error

esp_err_t rmt_receive(rmt_channel_handle_t rx_channel, void *buffer, size_t buffer_size, const rmt_receive_config_t *config)

Initiate a receive job for RMT RX channel.

备注

This function is non-blocking, it initiates a new receive job and then returns. User should check the received data from the on_recv_done callback that registered by rmt_rx_register_event_callbacks().

参数
  • rx_channel[in] RMT RX channel that created by rmt_new_rx_channel()

  • buffer[in] The buffer to store the received RMT symbols

  • buffer_size[in] size of the buffer, in bytes

  • config[in] Receive specific configurations

返回

  • ESP_OK: Initiate receive job successfully

  • ESP_ERR_INVALID_ARG: Initiate receive job failed because of invalid argument

  • ESP_ERR_INVALID_STATE: Initiate receive job failed because channel is not enabled

  • ESP_FAIL: Initiate receive job failed because of other error

esp_err_t rmt_rx_register_event_callbacks(rmt_channel_handle_t rx_channel, const rmt_rx_event_callbacks_t *cbs, void *user_data)

Set callbacks for RMT RX channel.

备注

User can deregister a previously registered callback by calling this function and setting the callback member in the cbs structure to NULL.

备注

When CONFIG_RMT_ISR_IRAM_SAFE is enabled, the callback itself and functions called by it should be placed in IRAM. The variables used in the function should be in the SRAM as well. The user_data should also reside in SRAM.

参数
  • rx_channel[in] RMT generic channel that created by rmt_new_rx_channel()

  • cbs[in] Group of callback functions

  • user_data[in] User data, which will be passed to callback functions directly

返回

  • ESP_OK: Set event callbacks successfully

  • ESP_ERR_INVALID_ARG: Set event callbacks failed because of invalid argument

  • ESP_FAIL: Set event callbacks failed because of other error

Structures

struct rmt_rx_event_callbacks_t

Group of RMT RX callbacks.

备注

The callbacks are all running under ISR environment

备注

When CONFIG_RMT_ISR_IRAM_SAFE is enabled, the callback itself and functions called by it should be placed in IRAM. The variables used in the function should be in the SRAM as well.

Public Members

rmt_rx_done_callback_t on_recv_done

Event callback, invoked when one RMT channel receiving transaction completes

struct rmt_rx_channel_config_t

RMT RX channel specific configuration.

Public Members

int gpio_num

GPIO number used by RMT RX channel. Set to -1 if unused

rmt_clock_source_t clk_src

Clock source of RMT RX channel, channels in the same group must use the same clock source

uint32_t resolution_hz

Channel clock resolution, in Hz

size_t mem_block_symbols

Size of memory block, in number of rmt_symbol_word_t, must be an even

uint32_t invert_in

Whether to invert the incoming RMT channel signal

uint32_t with_dma

If set, the driver will allocate an RMT channel with DMA capability

uint32_t io_loop_back

For debug/test, the signal output from the GPIO will be fed to the input path as well

struct rmt_rx_channel_config_t::[anonymous] flags

RX channel config flags

struct rmt_receive_config_t

RMT receive specific configuration.

Public Members

uint32_t signal_range_min_ns

A pulse whose width is smaller than this threshold will be treated as glitch and ignored

uint32_t signal_range_max_ns

RMT will stop receiving if one symbol level has kept more than signal_range_max_ns

Functions

esp_err_t rmt_del_channel(rmt_channel_handle_t channel)

Delete an RMT channel.

参数

channel[in] RMT generic channel that created by rmt_new_tx_channel() or rmt_new_rx_channel()

返回

  • ESP_OK: Delete RMT channel successfully

  • ESP_ERR_INVALID_ARG: Delete RMT channel failed because of invalid argument

  • ESP_ERR_INVALID_STATE: Delete RMT channel failed because it is still in working

  • ESP_FAIL: Delete RMT channel failed because of other error

esp_err_t rmt_apply_carrier(rmt_channel_handle_t channel, const rmt_carrier_config_t *config)

Apply modulation feature for TX channel or demodulation feature for RX channel.

参数
  • channel[in] RMT generic channel that created by rmt_new_tx_channel() or rmt_new_rx_channel()

  • config[in] Carrier configuration. Specially, a NULL config means to disable the carrier modulation or demodulation feature

返回

  • ESP_OK: Apply carrier configuration successfully

  • ESP_ERR_INVALID_ARG: Apply carrier configuration failed because of invalid argument

  • ESP_FAIL: Apply carrier configuration failed because of other error

esp_err_t rmt_enable(rmt_channel_handle_t channel)

Enable the RMT channel.

备注

This function will acquire a PM lock that might be installed during channel allocation

参数

channel[in] RMT generic channel that created by rmt_new_tx_channel() or rmt_new_rx_channel()

返回

  • ESP_OK: Enable RMT channel successfully

  • ESP_ERR_INVALID_ARG: Enable RMT channel failed because of invalid argument

  • ESP_ERR_INVALID_STATE: Enable RMT channel failed because it’s enabled already

  • ESP_FAIL: Enable RMT channel failed because of other error

esp_err_t rmt_disable(rmt_channel_handle_t channel)

Disable the RMT channel.

备注

This function will release a PM lock that might be installed during channel allocation

参数

channel[in] RMT generic channel that created by rmt_new_tx_channel() or rmt_new_rx_channel()

返回

  • ESP_OK: Disable RMT channel successfully

  • ESP_ERR_INVALID_ARG: Disable RMT channel failed because of invalid argument

  • ESP_ERR_INVALID_STATE: Disable RMT channel failed because it’s not enabled yet

  • ESP_FAIL: Disable RMT channel failed because of other error

Structures

struct rmt_carrier_config_t

RMT carrier wave configuration (for either modulation or demodulation)

Public Members

uint32_t frequency_hz

Carrier wave frequency, in Hz, 0 means disabling the carrier

float duty_cycle

Carrier wave duty cycle (0~100%)

uint32_t polarity_active_low

Specify the polarity of carrier, by default it’s modulated to base signal’s high level

uint32_t always_on

If set, the carrier can always exist even there’s not transfer undergoing

struct rmt_carrier_config_t::[anonymous] flags

Carrier config flags

Functions

esp_err_t rmt_new_bytes_encoder(const rmt_bytes_encoder_config_t *config, rmt_encoder_handle_t *ret_encoder)

Create RMT bytes encoder, which can encode byte stream into RMT symbols.

参数
  • config[in] Bytes encoder configuration

  • ret_encoder[out] Returned encoder handle

返回

  • ESP_OK: Create RMT bytes encoder successfully

  • ESP_ERR_INVALID_ARG: Create RMT bytes encoder failed because of invalid argument

  • ESP_ERR_NO_MEM: Create RMT bytes encoder failed because out of memory

  • ESP_FAIL: Create RMT bytes encoder failed because of other error

esp_err_t rmt_new_copy_encoder(const rmt_copy_encoder_config_t *config, rmt_encoder_handle_t *ret_encoder)

Create RMT copy encoder, which copies the given RMT symbols into RMT memory.

参数
  • config[in] Copy encoder configuration

  • ret_encoder[out] Returned encoder handle

返回

  • ESP_OK: Create RMT copy encoder successfully

  • ESP_ERR_INVALID_ARG: Create RMT copy encoder failed because of invalid argument

  • ESP_ERR_NO_MEM: Create RMT copy encoder failed because out of memory

  • ESP_FAIL: Create RMT copy encoder failed because of other error

esp_err_t rmt_del_encoder(rmt_encoder_handle_t encoder)

Delete RMT encoder.

参数

encoder[in] RMT encoder handle, created by e.g rmt_new_bytes_encoder()

返回

  • ESP_OK: Delete RMT encoder successfully

  • ESP_ERR_INVALID_ARG: Delete RMT encoder failed because of invalid argument

  • ESP_FAIL: Delete RMT encoder failed because of other error

esp_err_t rmt_encoder_reset(rmt_encoder_handle_t encoder)

Reset RMT encoder.

参数

encoder[in] RMT encoder handle, created by e.g rmt_new_bytes_encoder()

返回

  • ESP_OK: Reset RMT encoder successfully

  • ESP_ERR_INVALID_ARG: Reset RMT encoder failed because of invalid argument

  • ESP_FAIL: Reset RMT encoder failed because of other error

Structures

struct rmt_encoder_t

Interface of RMT encoder.

Public Members

size_t (*encode)(rmt_encoder_t *encoder, rmt_channel_handle_t tx_channel, const void *primary_data, size_t data_size, rmt_encode_state_t *ret_state)

Encode the user data into RMT symbols and write into RMT memory.

备注

The encoding function will also be called from an ISR context, thus the function must not call any blocking API.

备注

It’s recommended to put this function implementation in the IRAM, to achieve a high performance and less interrupt latency.

Param encoder

[in] Encoder handle

Param tx_channel

[in] RMT TX channel handle, returned from rmt_new_tx_channel()

Param primary_data

[in] App data to be encoded into RMT symbols

Param data_size

[in] Size of primary_data, in bytes

Param ret_state

[out] Returned current encoder’s state

Return

Number of RMT symbols that the primary data has been encoded into

esp_err_t (*reset)(rmt_encoder_t *encoder)

Reset encoding state.

Param encoder

[in] Encoder handle

Return

  • ESP_OK: reset encoder successfully

  • ESP_FAIL: reset encoder failed

esp_err_t (*del)(rmt_encoder_t *encoder)

Delete encoder object.

Param encoder

[in] Encoder handle

Return

  • ESP_OK: delete encoder successfully

  • ESP_FAIL: delete encoder failed

struct rmt_bytes_encoder_config_t

Bytes encoder configuration.

Public Members

rmt_symbol_word_t bit0

How to represent BIT0 in RMT symbol

rmt_symbol_word_t bit1

How to represent BIT1 in RMT symbol

uint32_t msb_first

Whether to encode MSB bit first

struct rmt_bytes_encoder_config_t::[anonymous] flags

Encoder config flag

struct rmt_copy_encoder_config_t

Copy encoder configuration.

Type Definitions

typedef struct rmt_encoder_t rmt_encoder_t

Type of RMT encoder.

Enumerations

enum rmt_encode_state_t

RMT encoding state.

Values:

enumerator RMT_ENCODING_COMPLETE

The encoding session is finished, the caller can continue with subsequent encoding

enumerator RMT_ENCODING_MEM_FULL

The encoding artifact memory is full, the caller should return from current encoding session

Structures

struct rmt_tx_done_event_data_t

Type of RMT TX done event data.

Public Members

size_t num_symbols

The number of transmitted RMT symbols (only one round is counted if it’s a loop transmission)

struct rmt_rx_done_event_data_t

Type of RMT RX done event data.

Public Members

rmt_symbol_word_t *received_symbols

Point to the received RMT symbols

size_t num_symbols

The number of received RMT symbols

Type Definitions

typedef struct rmt_channel_t *rmt_channel_handle_t

Type of RMT channel handle.

typedef struct rmt_sync_manager_t *rmt_sync_manager_handle_t

Type of RMT synchronization manager handle.

typedef struct rmt_encoder_t *rmt_encoder_handle_t

Type of RMT encoder handle.

typedef bool (*rmt_tx_done_callback_t)(rmt_channel_handle_t tx_chan, rmt_tx_done_event_data_t *edata, void *user_ctx)

Prototype of RMT event callback.

Param tx_chan

[in] RMT channel handle, created from rmt_new_tx_channel()

Param edata

[in] RMT event data

Param user_ctx

[in] User registered context, passed from rmt_tx_register_event_callbacks()

Return

Whether a high priority task has been waken up by this callback function

typedef bool (*rmt_rx_done_callback_t)(rmt_channel_handle_t rx_chan, rmt_rx_done_event_data_t *edata, void *user_ctx)

Prototype of RMT event callback.

Param rx_chan

[in] RMT channel handle, created from rmt_new_rx_channel()

Param edata

[in] Point to RMT event data

Param user_ctx

[in] User registered context, passed from rmt_rx_register_event_callbacks()

Return

Whether a high priority task has been waken up by this function

Unions

union rmt_symbol_word_t
#include <rmt_types.h>

The layout of RMT symbol stored in memory, which is decided by the hardware design.

Public Members

unsigned int duration0

Duration of level0

unsigned int level0

Level of the first part

unsigned int duration1

Duration of level1

unsigned int level1

Level of the second part

struct rmt_symbol_word_t::[anonymous] [anonymous]
unsigned int val

Equivalent unsigned value for the RMT symbol

Type Definitions

typedef soc_periph_rmt_clk_src_t rmt_clock_source_t

RMT group clock source.

备注

User should select the clock source based on the power and resolution requirement

1

Different ESP chip series might have different number of RMT channels. Please refer to the [TRM] for details. The driver won’t forbid you from applying for more RMT channels, but it will return error when there’s no hardware resources available. Please always check the return value when doing Resource Allocation.

2

Callback function (e.g. rmt_tx_event_callbacks_t::on_trans_done) and the functions invoked by itself should also reside in IRAM, users need to take care of this by themselves.